Urine specific gravity: concentration, hydration and how it differs from osmolality

Specific gravity (SG) compares the weight of a volume of urine with the same volume of water. The more waste and dissolved substances in the urine, the higher the figure. GPnotebook says urine SG shows the concentration of urine and represents the hydration status of the patient. Lab Tests Online UK lists the visual examination of urine, which evaluates colour, clarity and concentration, as the first part of urinalysis, and the chemical dipstick adds SG as one of its readings.

Specific gravity and urine osmolality both describe concentration, but osmolality counts particles and is measured directly, whereas SG reflects particle weight and is easily influenced by large molecules such as glucose and protein.

It is part of a routine urinalysis done at admission, before surgery, in pregnancy checks, and when symptoms suggest a urinary or kidney problem. The result shows whether the kidneys are able to concentrate urine when the body needs to save water and dilute it when there is too much. It is used as a quick indicator of hydration, and as a clue in the assessment of people who are passing unusual volumes of urine.

A first morning sample is the most useful as it is concentrated; Lab Tests Online UK says this makes abnormalities more likely to show up.

GPnotebook says normal specific gravity varies from 1.001 to 1.035. Causes of increased SG include conditions causing dehydration, glycosuria, renal artery stenosis, heart failure (secondary to decreased blood flow to the kidneys), inappropriate antidiuretic hormone secretion and proteinuria. Causes of decreased SG include excessive fluid intake, renal failure, pyelonephritis and diabetes insipidus.

In diabetes insipidus, there is too little ADH or a failure of kidney response, leading to large volumes of dilute urine, which Lab Tests Online UK says is diagnosed with a water deprivation test under specialist supervision.

GPnotebook says some dipsticks give falsely high readings in the presence of dextran solutions and IV radiopaque dyes, although this varies with different dipsticks, and that alkaline urine, for example from a high-citrate diet, may cause falsely low readings. It also notes that the usefulness of SG in identifying dehydration in infants has been brought into question, citing a paediatric emergency study that described urine specific gravity and other indices as inaccurate tests for dehydration.

Because of these limits, if a precise assessment of concentration is needed, doctors request urine osmolality paired with serum osmolality and sodium, as Lab Tests Online UK describes. Dipstick SG is a screening clue, not a diagnosis.

Provide a fresh clean mid-stream sample. No preparation is needed unless instructed. Mention recent X-ray contrast, intravenous fluids, high fluid intake, diuretics, and diabetes, since these change the result. Do not force extra fluids before the test, as this will dilute the sample and may mask a concentrating problem.

A very high reading in an unwell person prompts rehydration and review of the cause. A persistently low or fixed reading, with large urine volumes or thirst, prompts blood sodium, glucose, kidney tests and urine and serum osmolality. Seek urgent help for severe thirst with passing very large volumes of urine, confusion or collapse; call 999 or attend A&E.